trombiculid

Scrub typhus

Although it is similar in presentation to other forms of typhus, it is caused by an agent in a different Genus, and is frequently classified separately from the other typhi.

Causes and geographical distribution

Scrub typhus is transmitted by trombiculid mites ("chiggers"), which are found in areas of heavy scrub vegetation. The bite of this mite leaves a characteristic black eschar that is useful to the doctor for making the diagnosis.

Scrub typhus is endemic to a part of the world known as the "tsutsugamushi triangle", which extends from northern Japan and far-eastern Russia in the north, to northern Australia in the south, and to Pakistan and Afghanistan in the west.

The precise incidence of the disease is unknown, as microbiological facilities are not available in much of its native range. In rural Thailand and in Laos, murine and scrub typhus accounts for around a quarter of all adults presenting to hospital with negative blood cultures. The incidence in Japan has fallen over the past few decades, probably due to decreasing exposure, and many prefectures report fewer than 50 cases per year.
It affects females more than males in Korea, but not in Japan.

Symptoms

Treatment

Without treatment, the disease is often fatal. The clinical trial evidence on which to base treatment is small, but the drug most commonly used is doxycycline; tetracycline and rifampicin are alternatives. Other drugs that may be effective are azithromycin and the fluoroquinolones, but there is no evidence on which to recommend their use. Chloramphenicol and rifampicin may be useful for infection in children (doxycycline is relatively contraindicated in children).

Vaccine

There are currently no licensed vaccines available in the US.

An early attempt to create a scrub typhus vaccine occurred in the United Kingdom in 1937 (with the Wellcome Foundation infecting around 300,000 cotton rats in a classified project called "Operation Tyburn", but the vaccine was not used. The first known batch of scrub typhus vaccine actually used to inoculate human subjects was despatched to India for use by Allied Land Forces, South-East Asia Command (A.L.F.S.E.A.) in June, 1945. By December, 1945, 268,000 cc. had been despatched. An attempt to verify the efficacy of the vaccine by using a placebo group for comparison was vetoed by the military commanders, who objected to the experiment.

History

Severe epidemics of the disease occurred among troops in Burma and Ceylon during World War II (WWII). Several members of the U.S. Army's 5307th Composite Unit (Merrill's Marauders) died of the disease; and in 1944, there were no effective antibiotics or vaccines available. The disease was also a problem for US troops stationed in Japan after WWII, and was variously known as "Shichitou fever" (by troops stationed in the Izu Shichitou islands) or as Hatsuka fever (Chiba prefecture).